Top Voted Answer

in the World of Balance Mog is an optional character, you can get him by going into the treasure house in Narsh and follow Lone Wolf up to the summit where tritoch is, watch Mog and Lone Wolf for about a minute and they will strugle and both fall to an edge. Help Mog up immidiatly ignoring Lone Wolf speaking about the Gold Hairpin. Mog will then join your team. If you want to get all of Mog's dances you must get him in the World of Balance and take him on either the Lete River or through the Serpent Trench to learn Water Rondo. there is no place in the World of Ruin to learn this skill.

There are only three characters that you are forced to take with you in the world of Ruin: Celes, Edgar, and Setzer. all other characters could be considered optional for the second half of the game. so I will outline how to get each of them for you

Sabin: go to Tzen, the event to recruit him will initiate immidiatly

Terra: go to Mobliz and go into the first house to initiate some dialogue and a battle with Phunbaba, once you are given the Fenrir Magicite leave Mobilz and complete a few other portions of the game, you must at least get the airship before returning. when you return check the house to th far left side of town and look for a stair well behind a bookshelf, complete the dialogue and Second battle with Phunbaba and Terra will join.

Cyan: Go to Miranda and speak to the girl untill she asks you to atach a letter to her Pidgeon outside. Follow the Pigeon to Zozo, buy some rust rid from the merchant walking around on the ground and open up the rusted shut door. complete Mount Zozo and Cyan will Join

Gau: simply fight some battles on the Veldt with only three members in your party and he will join you after one of these fights as if nothing happened.

Shadow: If you do not wait for him on the Floating Continent you cannot get him back. Period. If you did wait for him on the Floating Continent then go to the Veldt and complete the Cave on the Veldt obtaining all treasure on the way. Once you have fought the Behemoth bosses in the cave go to the colliseum and wage the Striker dagger that you found in the cave. once you defeat shadow in this fight he will join you.

Relm: Go to Jidore after completing the Cave on the Veldt, complete the level under Owser's House and Relm will join you.

Strago: Go to the Fanatics Tower with Relm on your team and Strago will join your team automatically.

Mog: Go to Narsh and go to the cave where all the moogles lived, mog will join you there. as a bonus check the wall right behind where he was standing for a great relic.

Umaro: First be sure to have Mog on your team. Fight Tritoch at the top of the mountain and then jump into the crevace and complete the cave, once you fight Umaro Mog will have him join you. if you already completed the cave without Mog just go back and bring Mog with you to have Umaro join.

Gogo: go to Triangle Island in the upper right corner of the Map and get into a battle with the Zone Eater, let it engulf your entire team, then complete the dungeon you awaken in.

Locke: Once you have enough people to make two complete parties. (or one good party and Mog with the Moogle Charm in the other) try to land the airship in the tiny patch of land in the centre of the star shaped mountain just north of Tzen. this will give you access to the Pheonix Cave. Complete this area to have Locke join.

Answers

Shadow is the only one you can permanently miss.

To keep him when you are about to jump off the floating continent select "Wait" when you are prompted to and wait there until the time runs out. Shadow will come and the game continues. Afterwards to get him back you must bet the weapon "Striker" in the colosseum.

The other two secret characters are Umaro and Gogo.

To get Umaro you must defeat Tritoch at the end of Narshe and jump down the hole left behind. After the end of the cave you can fight Umaro. If you have Mog he will talk to Umaro and join the party.

To find Gogo go to the triangle shaped island in the upper right corner of the map and fight enemies there until a large worm engulfs your whole party. Complete this extra dungeon to get Gogo

.And as for Gau:You need to buy some Dried Meat at Mobliz, then go into the Veldt. When you encounter Gau, give him some of the Dried Meat. He will join you after a scene, and the game explains you how to use his Leap/Rage command.